1. Field of the Invention
The present invention relates to a crank shaft support structure of an engine and, more particularly to a crank shaft support structure of an engine in which a belt converter is attached to an end portion of the crank shaft.
2. Description of the Related Art
In a vehicle that travels on rough terrain such as a straddle-type all terrain vehicle, a belt converter is attached to an engine for the purpose of facilitating speed change operation.
FIG. 4 shows an example of a crank shaft support structure of an engine of a straddle-type four wheel all terrain vehicle in which the belt converter is attached to an end portion of the crank shaft.
As shown in FIG. 4, in this engine, a crank shaft 102 penetrates through a wall portion of a crank chamber 101a of a crank case 101 and a wall portion of a chain chamber 101b of the crankcase 101. A pair of bearings 103, 104 are provided on the wall portion of the crank chamber 101a through which the crank shaft 102 penetrates, and a bearing 112 for the belt converter is provided on an outer wall portion 111 of the chain chamber 101b through which the crank shaft 102 penetrates. The crank shaft 102 is held by the pair of bearings 103, 104, and the bearing 112 for the belt converter such that the crank shaft 102 is rotatable with respect to the wall portion of the crank chamber 101a and the outer wall portion 111 of the chain chamber 101b. 
A large end portion 104a of a connecting rod 104 connected to a piston 105 is connected to a crank pin 106 of the crank shaft 102 by means of a bearing 107. A belt converter 108 is attached to a portion 102a of the crank shaft 102 that is protruded from the wall portion of the chain chamber 101b. The belt converter 108 is attached such that a main shaft 109 is connected to the end portion 102a of the crank shaft 102 and a drive pulley 110 is attached to the main shaft 109. The bearing 112 for the belt converter is constituted by a double row ball bearing so that it withstands belt tension of the belt converter 108 because the bearing 112 is subject to the belt tension.
An oil passage 115 is formed through an inside of the outer wall portion 111 of the chain chamber 101b, an inner peripheral face of the outer wall portion 111 through which the crank shaft 102 penetrates, and inside of the crank shaft 102, to a portion 120 where the crank shaft 102 and the large end portion of the connecting rod are connected to each other. Through the oil passage 115, lubricating oil is forcibly fed by an oil pump (not shown) to the portion 120 where the large end portion of the connecting rod and the crank shaft 102 are connected to each other. Reference numeral 114 denotes a sprocket for driving a cam shaft (not shown).
Publication of Japanese Examined Patent Application No. Hei. 2-29889 discloses a general crank shaft support structure of an engine in which a bearing for supporting a crank shaft is constituted by a plain bearing, and from the plain bearing, oil is forcibly fed to a portion where a large end portion of a connecting rod and a crank shaft are connected to each other.
By the way, when the conventional engine is mounted on the straddle-type four wheel all terrain vehicle, the crank shaft 102 is placed in a lateral direction of a vehicle body. The conventional engine is a single-cylinder engine, but a two-cylinder engine is sometimes mounted. In this case, a width of the engine is increased because the number of cylinders is increased as compared to the case where the single-cylinder engine is mounted. When a total width of the engine and the belt converter is too large in the straddle-type four wheel all terrain vehicle, a rider straddling a seat makes contact with side faces thereof. Therefore, it is necessary to limit the engine width.
However, in the crank shaft support structure of the conventional engine, a portion to be supported by the bearing 112 for the belt converter and a portion in which the oil passage 115 is formed are required in a portion of the crank shaft 102 that is outwardly protruded from a wall portion of the crank chamber 101a. For this reason, the length of the crank shaft 102, and hence the width of the engine, are increased.
Under the circumstances, an object of the present invention is to provide a crank shaft support structure which is capable of reducing a length of a crank shaft of an engine to which a belt converter is attached.
To achieve the above-described object, according to the present invention, there is provided a crank shaft support structure of an engine comprising: a crank shaft provided such that it penetrates through a crank case, including a crank pin portion situated in the crank case, to which a large end portion of a connecting rod is connected, and having one end portion to which a belt converter is connected; a first bearing for holding a portion of the crank shaft that penetrates through the crank case, which is close to the one end portion of the crank shaft, rotatably with respect to the crank case, the first bearing being constituted by a double row ball bearing; a second bearing for holding a portion of the crank shaft that penetrates through the crank case, which is close to the other end portion of the crank shaft, rotatably with respect to the crank case; and means for forcibly feeding lubricating oil from the other end portion""s side of the crank shaft, through an oil passage formed inside of the crank shaft, and to a portion where a large end portion of a connecting rod and the crank pin portion are connected to each other.
With this configuration, since the oil is fed to the portion where the large end portion of the connecting rod and the crank shaft are connected to each other, from an opposite side of the portion where the belt converter is provided, the belt converter can be provided adjacently to the first bearing. Also, a large load capacity is demanded of the bearing which is subjected to load from the belt converter, because it needs to withstand belt tension of the belt converter which is applied to the bearing in the radial direction thereof. Since the first bearing is constituted by the double row ball bearing with a load capacity large enough to withstand the belt tension, a bearing for the belt converter can be dispensed with. Consequently, the length of the crank shaft and hence, the width of the engine can be reduced.
In this case, the second bearing may be a plain bearing, the oil passage may be formed in the crank shaft such that the oil passage extends from a peripheral face of a portion of the crank shaft that is fittingly inserted into the second bearing, through the inside of the crank shaft, and to the portion where the large end portion of the connecting rod and the crank pin portion are connected to each other, and the means for forcibly feeding oil may be adapted to feed the lubricating oil to the second bearing, and forcibly feed the lubricating oil from the second bearing, through the oil passage, and to the portion where the large end portion of the connecting rod and the crank pin portion of the crank shaft are connected to each other. With this configuration, it is not necessary to provide an oil passage at a portion of the crank shaft that is situated outwardly of the second bearing. Consequently, the crank shaft can be further reduced.
These objects as well as other objects, features and advantages of the invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.